Commercial single junction silicon based solar cells have a theoretical conversion efficiency limit of 29%. Record efficiencies of approximately 25% have been demonstrated for laboratory-based solar cells.
Higher conversion efficiencies can be obtained by using stacks of solar cells of different materials on top of each other. To date the highest conversion efficiencies have been obtained using stacks of solar cells based on III-V semiconductor materials. Record devices comprising four stacked III-V based solar cells have reached a conversion efficiency of 44.7% at a concentration of 297 suns.
These III-V stacked solar cells have been employed in space applications for many years. More recently, they have been used in small concentrated solar power plant. The very high manufacturing cost of these cells prevents them from becoming a viable solution for major plants or residential photovoltaic systems.
To achieve very high conversion efficiencies, the III-V materials used to make the photovoltaic device must have an extremely low concentration of physical and electrical defects. In order to reach these defects concentrations the materials must be manufactured on crystalline germanium substrates or epitaxial germanium layers which are lattice matched to the III-V structure.
For example, crystalline germanium provides very good lattice parameters for the growth of a variety of III-V materials. Germanium substrates are used extensively to manufacture III-V based solar cells. However, these substrates are expensive to manufacture and often account for a third of the cost of the whole solar cell device. Epitaxial germanium layers, on the other hand, are grown by molecular beam epitaxy (MBE) or chemical vapour deposition (CVD) techniques. These techniques require high vacuum (5×10−11 mbar and 1.5×10−9 mbar respectively) and use toxic gases such as germane and silane.
One of the challenges to reduce the price of III-V based high efficiency solar cells has been to decrease the cost of the substrates while maintaining the lattice matched properties.